In September 2016 the European spacecraft Rosetta touched down softly on the surface of Comet 67P/Churyumov-Gerasimenko (Chury for short), crowning a more than two-year exploration of an unusual world.
What is a comet?
A comet is a mountain-sized block of ice and rock that circles the Sun in a very elongated orbit. As the comet approaches the Sun, the ice sublimates and envelops the comet’s nucleus in a coma of gas and dust. The solar wind stretches out the coma to create gas-and-dust tails, making comets one of the most beautiful astronomical phenomena that we get to observe.
Comets come from the outer regions of the solar system and are made up of the same primitive matter that went into the origin of the planets. Study of these bodies therefore provides us with information about planet formation processes, including the Earth’s.
What did we learn from the Rosetta mission?
Space exploration has made it possible to flyby a few comet nuclei, but the European Space Agency went further with the Rosetta mission. The goal was to orbit a probe around Comet 67P/Churyumov-Gerasimenko and to track it in its flight when it got to its closest point to the Sun. A lander, named Philae, was also integrated into the mission to directly explore the comet’s nucleus.
Launched in 2004, the spacecraft took ten years to reach its target. The first pictures revealed a strangely duck-shaped comet! Subsequent high-resolution images showed a nucleus with a complex geology rather than a smooth surface.
Chury would in fact prove to be two comet nuclei that got welded together as the result of a low-velocity collision. The diameter is about four kilometers for the big piece and two kilometers for the small one.
A risky landing
The gravity of this comet being very low, Philae had to latch on to the surface with the help of harpoons. Unfortunately, a malfunction of those harpoons caused Philae to bounce up twice before it could finally stabilize in a shaded crevasse. This led to the conclusion that, even though the nucleus is porous, its surface is very hard.
Comet Chury’s nucleus is extremely dark, a bit like coal. That can be explained by the fact that the surface is covered with several centimeters of organic matter. It’s estimated moreover that 40 percent of the comet is composed of that type of matter. In addition, a number of organic compounds have been identified on the comet, including glycine, one of the molecules that are essential to life on Earth.
More than 6 million tons of water lost in 2 years!
Rosetta taught us that ice sublimation does not happen uniformly on the nucleus surface, but comes from holes about a hundred meters in diameter. When struck by the light of the Sun, the matter on the surface heats up and then is expelled for several dozen minutes. When Chury passed closest to the Sun in August 2015, it was losing 100,000 tons of water a day!
That activity produces an erosion of the nucleus that quickly changes the geology of the comet.
Also observed was a fissure longer than one kilometer – which lengthened during the two years of the Rosetta mission – in the area between the comet’s two lobes. This suggests that one day, perhaps not very far off, Chury will return to being two distinct comet nuclei.
In 2017 an English scent company developed an “eau de comète” reproducing the smells given off by the celestial body: a mixture of rotten eggs, cat urine and almonds past their best-by date. Despite its eccentricity, the perfume hasn’t enjoyed much success!
To learn more about the Rosetta mission, I invite you to the Planétarium to see the immersive film The Adventures of Rosetta and Philae.